In a total reflection measuring method (ATR method), when an infrared light is totally reflected at a contact surface between a sample and a prism (also referred to as an ATR crystal), the light enters a surface part of the sample slightly. And the light is absorbed and reflected by the surface part of the sample. The ATR method is a method for measuring an absorption spectrum of the surface part of the sample from its total reflection light. For example, the ATR method is widely used in surface analysis of polymer membranes, semi-conductors, or the like, and analysis of samples that remarkably absorb infrared light such as aqueous solution.
In the ATR method, the sample is placed upside-down on the prism, and the sample and the prism are in a contact state. Therefore, there was a need for grasping which part of the sample is actually being measured in a visible observation image by observing the contact region where total reflection occurs. It is more preferable if a visible observation and a spectrum measurement can be performed simultaneously.
Patent Literature 1 describes a prism retaining base 90 for the ATR that comprises an optical system for visual observation that uses an infrared light, not a visible light. As shown in FIG. 5, the prism retaining base 90 comprises: a prism 92 that is supported to a housing 91 and transmits an infrared light; a plane mirror 93 that makes an infrared light IR for measuring a spectrum from a light source incident to the prism 92; and a plane mirror 96 that makes a total reflection light exit at a contact surface 94 of the prism to a side of a detector 95 for measuring the spectrum. Further, a half mirror 97 for observing the sample is provided in a direction perpendicular to the contact surface 94 of the prism. FIG. 5 shows a state that the sample on a sample base in contact with the prism 92. In this state, the half mirror 97 reflects the infrared light for observation from the light source for observation to the contact surface 94 of the prism, and transmits a reflected light from the contact surface 94 to an imaging element 98 for observing the sample. That is, the infrared light for observation is irradiated from right above the prism 92 as an illumination light, and the state of the sample is observed by a monitor or the like based on the reflected light from the contact surface 94.